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Nanoplastics migration in hydroxyapatite-amended porous media: Integration of XDLVO theory and two-site kinetic modeling
Summary
Researchers ran column transport experiments showing that nanoplastic particles migrate far less through hydroxyapatite-amended soils than through plain quartz sand — penetration rates dropped from 97% to just 1.3% as hydroxyapatite content increased from 0.1% to 1% — with divalent calcium ions and organic acids further reducing mobility, informing risk assessment for plastic-contaminated soils amended with hydroxyapatite.
This study systematically investigates the transport of carboxyl-modified polystyrene nanoparticles (CPSNPs) in saturated hydroxyapatite (HAP)-quartz sand (QS) porous media through column experiments. The research examines the influence of hydroxyapatite (HAP) mass fraction, ionic types (Na, Ca, HPO), ionic strength, and concentrations of organic acids (oxalic acid, humic acid) and nanocellulose (CNC). Migration mechanisms were elucidated using the extended DLVO theory and a two-site kinetic model. Results demonstrate that increasing the HAP mass fraction from 0.1% to 1% enhances aggregation between HAP and CPSNPs, forming larger aggregates that effectively trap more CPSNPs via increased adsorption sites and pore blocking. This phenomenon leads to a significant reduction in the penetration rate from 97.30% to 1.30%. Monovalent (Na) and divalent (Ca) cations inhibit CPSNPs transport, whereas anions (HPO) promote CPSNPs mobility. Ca exerts stronger inhibition due to more effective charge screening. Both humic acid and oxalic acid reduce CPSNPs mobility, with oxalic acid exhibiting more pronounced inhibition. CNC enhances transport at concentrations below 60 mg·L but inhibits it above this threshold. The chemical non-equilibrium two-site model provides a good fit to the experimental data (R > 0.949). This study elucidates the transport rules of CPSNPs in HAP-QS media, offering a scientific basis for risk assessment of nanoplastics in HAP-amended soil-groundwater systems and practical guidance for evaluating the impact of hydrochemical conditions on HAP's nanoplastics capture efficiency. The kinetic parameters obtained also offer critical insights for the in-situ remediation of soils co-contaminated by heavy metals and nanoplastics.